1. Field of the Invention
The present invention relates to a parallel combinatory code-division multiple access (PC-CDMA) system that transmits and receives data by applying a predetermined pseudo number (PN) code corresponding to a plurality of bits, and more particularly relates to a multi-carrier PC-CDMA system that enhances its efficiency by mixing the PN codes corresponding to the plurality of bits with respective orthogonal frequencies.
2. Discussion of Related Art
Recently, according as various communication equipments have applied wireless and digital increasingly, communication systems have applied a code-division multiple access (CDMA) gradually, instead of a time-division multiple access (TDMA) or a frequency-division multiple access (FDMA), and more detailedly a direct sequence CDMA (DC-CDMA) and a parallel combinatory CDMA (PC-CDMA) system, etc. have been commonly used. Especially, the PC-CDMA system, which can minimize its complexity and at the same time transmit a great deal of information at high speed, is being watched with keen interest as a mobile communications system such as cellular system.
FIG. 1a and 1b are block diagrams explaining an outline of the PC-CDMA system that transmits and receives, for example, 128-bit data, FIG. 1a shows a transmitting part and FIG. 1b a receiving part. In FIG. 1a, reference numeral 11 denotes a serial-to-parallel converter converting a series of serial data into that of parallel data, reference numerals 12.sub.1 to 12.sub.32 denote mappers receiving predetermined bits (for example, 4-bit) of data from the serial-to-parallel converter 11 and outputting PN codes corresponding to the bit values and reference numeral 13 denotes a multiplexer mixing the PN codes from the mappers 12.sub.1 to 12.sub.32 to output. The PN codes outputted from the multiplexer 13 are digital-modulated by means of phase shift keying (PSK) or quadrature PSK (QPSK) and then transmitted through a radio communications network.
In FIG. 1b, reference numerals 21.sub.1 to 21.sub.31 denote correlators for outputting level signals corresponding to the PN codes of data received and demodulated from the transmitting part. FIG. 2 illustrates a configuration of the correlators 21.sub.1 to 21.sub.31 of FIG. 1b, comprising a plurality of multiplicators 211.sub.1 to 211.sub.16 mixing received data with respective intrinsic PN codes [PN(1), PN(2), . . . , PN (15) and PN(16)] and a plurality of integrators 212.sub.1 to 212.sub.16 integrating signals outputted from the multiplicators 211.sub.1 to 211.sub.16.
In FIG. 1b, reference numerals 22.sub.1 to 22.sub.32 denote data detectors detecting PN codes, transmitted from the transmitting parts of FIG. 1a, based on the level signals inputted from the correlators 21.sub.1 to 21.sub.31. Reference numeral 23 denotes a parallel-to-serial convertor converting parallel data from the data detectors 22.sub.1 to 22.sub.32 into serial data.
The mappers 12.sub.1 to 12.sub.32 of the transmitting part output PN codes corresponding to 4-bit data values inputted, the PN codes from the mappers 12.sub.1 to 12.sub.32 are mixed by multiplexer 13 and digital-modulated to be transmitted. Here, as the mappers 12.sub.1 to 12.sub.32 output respective intrinsic PN codes corresponding to the 4-bit data values inputted, all data outputted from the mappers 12.sub.1 to 12.sub.32 have orthogonal feature.
In the receiving part, each of the correlators 21.sub.1 to 21.sub.32 mixes received and demodulated data with all the PN codes [PN(1), PN(2), . . . , PN(511) and PN(512)] which are to be transmitted from the transmitting part and then integrates respective data to output, and each data detector 22.sub.1 to 22.sub.32 detects the PN codes of received data, based on 512-signal level outputted from the correlators 21.sub.1 to 21.sub.32, and restores 128-bit data (respectively 4-bit) to output. Accordingly, in the above configuration, data can be transmitted and received at high speed as 128-bit data are transmitted and received at the same time.
However, the above described PC-CDMA system has the following problems. When transmitting 128-bit data, 512-PN code (16.times.32) are needed as shown in FIG. 1 as 16-PN code (2.sup.4) are needed when transmitting 4-bit data in a conventional PC-CDMA system. At least 256-PN code are needed and even plus (+) PN codes and minus (-) PN codes are applied to decrease the number of PN code. Accordingly, a PN code generator becomes complicated to generate a large number of PN codes, which prevents diminishing the size of cellular system. The above described PC-CDMA system deteriorates data restoration at the receiving part due to the large number of PN codes. That is, when denoting a signal energy transmitted as an Eb and a noise as a No, the data restoration in a CDMA system is expressed as Eb/No in general, here, all the other PN codes except relevant PN codes act as noise signals. Therefore, in a cellular system using the above described PC-CDMA system, when a great deal of users try to make phone calls simultaneously, PN codes transmitted and received from and to the other cellular phones act as noise signals, which increase the whole noise level, thus all the users cannot make phone calls.
Furthermore, in a PC-CDMA system as described above, considering the receiving part needs correlators 21.sub.1 to 21.sub.32 and data detectors 22.sub.1 to 22.sub.32 as much as the number of the mappers 12.sub.1 to 12.sub.32 needed at the transmitting part, it is an obstacle to minimize a portable communications terminal where the PC-CDMA system is applied.